Vehicle seat occupancy detection systems are nowadays widely used in vehicles, in particular in passenger cars, for providing a seat occupancy signal for various appliances, for instance for the purpose of a seat belt reminder (SBR) system or an activation control for an auxiliary restraint system (ARS). Seat occupancy detection systems include seat occupancy sensors that are known to exist in a number of variants, e.g. based on capacitive sensing.
Further, vehicle seat occupancy detection systems are known to be employed as a means of assessing a potential activation of an installed vehicle passenger restraint system, such as an airbag. An output signal of the seat occupancy detection system is usually transferred to an electronic control unit of the vehicle to serve, for instance, as a basis for a decision to deploy an air bag system to the vehicle seat.
It is further known in the art to use a portion of an electric seat heater of a vehicle seat as a sensor in a capacitive seat occupancy detection system. For example, patent application publication DE 41 10 702 A1 describes a vehicle seat with an electric seat heater comprising a conductor which can be heated by the passage of electrical current through it. The conductor is located in the seating surface and forms a part of a capacitive sensor for detecting a seat occupancy of the seat.
Seat occupancy detection systems based on capacitive sensing are based on determining a change in complex impedance Z or admittance Y of at least one sensing electrode of a capacitive sensor that is caused by an object approaching the sensing electrode. A seat occupancy detection or a seat occupancy classification may be determined by comparison of a measured complex impedance Z or admittance Y of the sensing electrode with predetermined threshold values and/or predetermined tolerance ranges.
The predetermined threshold values and/or predetermined tolerance ranges are usually defined for dry ambient conditions in the surroundings of the sensing electrode, as it is reasonable to presume that there is no liquid water present in the seat. The term “dry”, as used in this application, shall particularly be understood as an absence of liquid water and shall encompass a presence of water vapor in a range of 0% to 100% relative humidity. The term “wet”, as used in this application, shall in particular be understood as a condition encompassing a presence of liquid water.
However, situations exist in which a seat or a portion of the seat could get wet. This may happen, for instance, if a vehicle seat occupant spills a liquid, such as water or coffee, on the seat.
Wth portions of the seat having gotten wet that are arranged close to the sensing electrode, the measured complex impedance Z or admittance Y of the sensing electrode can substantially be changed, even so much so that the comparison of a measured complex impedance Z or admittance Y with the predetermined threshold values and/or predetermined tolerance ranges can lead to a wrong assessment concerning seat occupancy conditions or seat occupancy classes. As the liquid water distributes within the seat over time, the effect on the measured complex impedance Z or admittance Y of the sensing electrode also varies in an unpredictable manner.